1. Field of the Invention
Embodiments of the present invention generally relate to the fabrication of integrated circuits and to the fabrication of photomasks useful in the manufacture of integrated circuits.
2. Description of the Related Art
In the manufacture of integrated circuits (IC), or chips, patterns representing different layers of the chip are created by a chip designer. A series of reusable masks, or photomasks, are created from these patterns in order to transfer the design of each chip layer onto a semiconductor substrate during the manufacturing process. Mask pattern generation systems use precision lasers or electron beams to image the design of each layer of the chip onto a respective mask. The masks are then used much like photographic negatives to transfer the circuit patterns for each layer onto a semiconductor substrate. These layers are built up using a sequence of processes and translate into the tiny transistors and electrical circuits that comprise each completed chip. Thus, any defects in the mask may be transferred to the chip, potentially adversely affecting performance. Defects that are severe enough may render the mask completely useless. Typically, a set of 15 to 30 masks is used to construct a chip and can be used repeatedly.
A mask is typically a glass or a quartz substrate that has a layer of chromium on one side. The mask may also contain a layer of silicon nitride (SiN) doped with molybdenum (Mo), or alternating pairs of molybdenum (Mo) and silicon (Si) layers. The chromium layer is covered with an anti-reflective coating and a photosensitive resist. During a patterning process, the circuit design is written onto the mask by exposing portions of the resist to ultraviolet light, making the exposed portions soluble in a developing solution. The soluble portion of the resist is then removed, allowing the exposed underlying chromium to be etched. The etch process removes the chromium and anti-reflective layers from the mask at locations where the resist was removed, i.e., the exposed chromium is removed.
During the etching process, a plasma is used to enhance a chemical reaction and etch the exposed chromium and quartz area of the mask. Undesirably, conventional chromium and quartz etch processes often exhibit poor control of the etching endpoint, resulting in over-etching to the underlying quartz while patterning the chromium layer. Over-etching of the chromium layer results in damage to the underlying quartz, leading to an offset height of the quartz at different locations exposed by the patterned chromium layer. FIGS. 1A-1C depict an example of an etching process conventionally available and used to etch chromium and quartz. As shown in FIG. 1A, a quartz substrate 102 may include a chromium layer 104 having a patterning photoresist layer 106 disposed thereon. Openings 108 are formed in the photoresist layer 106 to expose a surface 110 of the chromium layer 104 for etching. Subsequently, an etching process is performed to etch the chromium layer 104 exposed by the openings 108 to expose the underlying quartz substrate 102. However, poor control of the etching process often result in over-etching deep to the underlying quartz substrate 102, undesirably removing a portion 114 of the quartz from the substrate 102, as shown in FIG. 1B. The over-etched portion 114 of the quartz substrate 102 results in an offset from the target etched surface 112, creating a different starting point of the quartz substrate 102 to the subsequent quartz substrate etching process. Accordingly, after performing the quartz etching process, the openings 108 formed in the quartz substrate 102 may have a different etching depth 118, 120 than the desired depth, thereby resulting in inaccurate feature transfer, as shown in FIG. 1C, and also correspondingly diminishing the ability to produce features having small critical dimensions using the mask.
As the critical dimensions of masks continue to shrink, the importance of uniform etch result increases. Thus, a quartz etch process having precise etching depth and dimension control is highly desirable.
Therefore, there is an ongoing need for an improved etching endpoint process control in photomask fabrication, including improved apparatus and methods for determining process endpoints.